In the art, there are known several nucleic acid mimics having nucleobases bound to backbones other than the naturally occurring ribonucleic acid or deoxyribonucleic acid backbones having the ability to bind to nucleic acids having a nucleobase sequence complementary to the base sequence of the nucleic acid mimic. Among these, only the peptide nucleic acids (PNA's) as described, for example, in WO 92/20702 have demonstrated a likelihood for potential use as therapeutic and diagnostic reagents. This may be due to their ability to bind nucleic acids (NAs) of complementary nucleobase sequence with a higher affinity than shown by the corresponding wild-type nucleic acid.
One of the unique properties of PNAs is their ability to form PNA2-NA triplexes that are more stable than the corresponding PNA-NA duplexes. This ability can be used advantageously for various purposes including PCR clamping (WO 93/25706). However, there are some drawbacks for applications that require sequence selection, because such selection would be biased for triplex forming sequences. Therefore, there is a need for PNAs that do not form such triplexes.